Pre-adjusted prosthetic ligament and method of manufacture

ABSTRACT

A prosthetic ligament and its method of manufacture is provided by which the prosthetic ligament is comprised of a plurality of strands aligned in a parallel lengthwise relationship to form a bundle, with the strands being interwoven at their respective ends while the middle portion of the strands remain unattached to adjacent strands. A predetermined torsion is applied to each strand prior to forming the bundle to create natural coiling in either a clockwise or counter-clockwise orientation. The prosthetic ligament may be used for a posterior or anterior cruciate ligament of the knee joint.

The invention relates to a left or right-oriented prosthetic ligament toreplace a biological joint ligament, and to several methods for makingsuch pre-oriented ligaments; these left-gyratory or right-gyratoryligaments can be especially used in knee plasty to replace anterior orposterior cruciate ligaments whether for left or right knee joints.

A certain number of artificial ligaments intended to replace jointligaments are already known to the prior art. Such ligaments includethose obtained by convoluted folding or rolling of fabric strips inbiocompatible material, whether woven or knitted such as to leave free,or on the contrary join together, the technical longitudinal strands ofthe ligament.

This is particularly the case for the ligaments described in Frenchpatent FR-2 697 151 filed by the applicant which, in addition to thehigh dynamometric properties of previous prostheses, provide for furtherimproved response of ligaments to the demands placed upon them afterimplant, through their close approximation of the geometry andconfiguration of the anatomic ligaments. For this purpose, an artificialligament was provided, comprising two intra-osseous ends, and anintra-articular intermediate part, characterised in that theintra-articular part was formed of two independent cords of longitudinalfibres obtained by convoluted rolling of at least one knitted width offabric, partly slit at its median portion over a distance at least equalto that of the intra-articular part, both intra-osseous ends beingtwisted by at least one quarter of a turn in relation to one anotherwhen the ligament is in place within the joint.

It is known that this type of ligament offers qualities that are relatedto the twin-cord structure which, in particular for the replacement ofthe anterior cruciate ligament of the knee, provides an undeniablebiomechanical advantage since it has been well proven that the anteriorcruciate ligament undergoes more than 45° torsion when the knee movesfrom maximum flexion to maximum extension; therefore, by implanting anartificial ligament that is pre-oriented in the same direction as thisnatural torsion movement between the tibia and the internal femoralcondyle, on a flexed knee, it is possible to cause the two cords formingthe ligament to untwist when the knee is in extension, and they thendisplay a perfectly parallel structure.

With this possibility, the longitudinal strands which make up thetechnical core of the ligament become parallel to one another when theknee is close to extension, that is to say in the position in which itusually suffers trauma when sprained; when stresses are applied, demandis placed upon these strands in progressive manner forming a trueshock-absorbing system that reproduces the anatomical process.

Nonetheless, it is known that this effect can only be truly substantialwith an artificial ligament if there are a sufficient number oflongitudinal strands which can be activated on demand to surround theisometric neutral fibre as closely as possible, which implies aprosthetic construction comprising the greatest number of strands in thesmallest cylindrical volume; it is recalled in passing that the neutralfibre of a ligament is the one which passes through the isometric axisjoining together the isometric points of the bones of the jointinvolved.

It is easy to understand that in a twin-bundle configuration such asprovided by the prior art, it is difficult to achieve instant responsefrom a sufficient number of longitudinal strands around the neutralfibre.

It therefore appeared that an artificial ligament such as described inFrench patent FR 2 688 690 by the same applicant, characterised in thatits intra-articular part is solely made up of adjacent strands that arenot connected together, could advantageously achieve this effect;according to the teachings given by this prior document, the ligamentused is made from a width of fabric which preserves the parallelism ofthe adjacent strands which can consequently resist the efforts appliedto the implanted ligaments, in line with their own longitudinaldirection.

On account of its intra-articular structure that is non-woven,non-knitted and more generally has no weft, it is possible toconsiderably increase the number of active strands while maintaining thesame volume capacity as a ligament that is knitted or woven over itsentire length; in this way it is virtually possible to double the numberof active strands of the ligament.

Also, this more compact structure of the active strands brings thelatter closer to the “neutral fibre” of the ligament.

The drawback of this type of so-called “free-strand” ligament is that itcannot be naturally coiled around itself in its intra-articular medianportion, as can the twin-bundle ligament described in French patent FR 2697 151 already described.

The purpose of the present invention is precisely to provide a “freestrand” ligament that is naturally self-convoluted between the two endsof the intra-articular median part.

In this respect, the invention provides a method of producing aprosthetic ligament for the replacement of a natural joint ligament,comprising between two end parts a median part that is solely made up ofa bundle of so-called “active” strands in mono or multi-filament of 1200decitex or less, that are longitudinal, adjacent and not bound to oneanother in crosswise direction, characterised in that prior to theassembly of the ligament a longitudinal torsion is imparted to eachactive strand, a clockwise or anti-clockwise direction of torsion beingchosen so as to form respectively a prosthetic ligament for right jointor left joint, the ends of the active strands being held together by anyappropriate means over a sufficient distance to form the end parts ofthe prosthetic ligament intended to be embedded into the bone tunnels ofthe joint.

All the advantages of this method are easy to understand since themanner described below, in which sufficient torsion is imparted to eachactive strand, is all that is required for the resulting torque tocreate natural coiling of the ligament by at least one eighth of a turnbetween the two ends of the active median portion, corresponding inpractice to optimal torsion at least when the bony insertion tunnels areideally situated.

Bearing in mind also that in a knee flexed at 90°, the two bundles ofthe biological anterior cruciate ligament are coiled in clockwisedirection for the right knee and in anti-clockwise direction for theleft knee, and in accordance with an essential characteristic of theinvention, the direction of torsion of the active strands of theprosthesis is advantageously chosen between a clockwise or anticlockwisedirection, determining respectively a prosthetic ligament for a right orleft joint.

Other characteristics and advantages will be better understood from thefollowing description of several methods of imparting torsion to theactive strands according to whether a bundle of mono-filament ormulti-filament strands is used, at least in the intra-articular portionof the prosthetic ligament, this description being given forillustrative purposes and is non-restrictive with reference to thedrawings in which:

FIG. 1 is a diagram of the main stages of a first variant of the torsionmethod for the active strands in a continuous, parallel mono-filament ormulti-filament configuration,

FIG. 2 shows two diagrams of cable twisted threads able to impartnatural torsion to each assembly either in clockwise direction (diagramS) (FIG. 2 a) or anti-clockwise direction (diagram Z) (FIG. 2 b),

FIG. 3 is an elevation view of the prosthetic ligaments: left-gyratory(3 a) right-gyratory (3 b) and left-gyratory with two convoluted rolls(3 c).

With reference to FIG. 1 a first variant of the method of producingprosthetic ligament 1 will be described intended to replace a naturaljoint ligament such as for example the anterior or posterior cruciateligaments of the knee. Said prosthetic ligament 1 comprises two endparts 2, 2′ normally intended to be embedded in the bone insertiontunnels of the joint; around end parts 2, 2′, a median portion 3 isexclusively made up of a bundle of active strands 4; active strands 4,advantageously in a biocompatible polyester type material, may be eithermono-filament or multi-filament, that is to say made up of a certainnumber of continuous filaments placed parallel to one another to form asingle thread 4 which must be as thin as possible while remainingsufficient to resist the stresses to which each type of ligament issubjected; taking into consideration the tests conducted, the bestresults are obtained with polyester filaments having a maximum decitexvalue of 1200, such as for example the filaments carrying the referencePES TYPE 156 made by Rhône Poulenc.

For all that follows, median part 4 of ligament 1 corresponds mostprecisely to the intra-articular portion of the joint to be repaired,for example the knee.

Active strands 4 forming median part 3 of ligament 1 are held at theirupper and lower ends by an assembly of weft threads 5, 5′ connectingthem crosswise; thread assemblies 4, 5, 5′ make up the end parts 2 and2′ of the intra-osseous part of ligament 1.

This type of assembly is known from French patent FR 2 688 690 alreadydescribed in the preamble.

According to one variant of the method of the invention, ligament 1 ismade such that it naturally has an active median portion twisted arounditself at an angle a that is ideally 45° or greater, such as shown inthe diagram in FIG. 1 by references A and B vertically aligned on theleft side of FIG. 1 showing the step-by-step assembly of fabric width 6such as described below, and staggered at an angle a on the right sideshowing a finished ligament.

Taking into account that active strands 4 are unattached crosswise inmedian part 3 of the ligament, the latter cannot have a natural torqueA, B. Under these conditions, the invention provides that, at the timeof preparation of fabric width 6, a longitudinal torsion in the samedirection and of the same value is imparted to each active strand 4. Toconfer this torsion upon each active strand 4, said strands 4 areunwound axially starting from their free end 7 from fixed spools 8 whoseoriginal direction of winding will determine the final direction oftorsion of each strand 4.

It can be understood that since spools 8 dispensing strands 4 are fixed,and since end 7 of each strand 4 is pulled vertically and held withoutcompensation, the latter will coil around itself in a spiral whose pitchis equivalent to one turn of the spool, the diameter of the latterdetermining the torsion angle per unit length.

It is hence understood that it is easy to adjust the torque of eachstrand 4 by choosing the direction of winding of the strand on eachspool 8 whose diameter determines final torque value.

All strands 4 thus coiled being positioned parallel to one another andforming the warp of fabric width 6, all that is needed is a well-knownmanner to produce an assembly of strands 4 with the use of cross threads5, 5′ advantageously of the same quality, to form the weft of fabricwidth 6 at least on two side strips corresponding to the bone end parts2, 2′ of the ligament such as shown on the left of FIG. 1; it raises noproblem to produce a fabric width 6 using any technique known to personsskilled in the art, for example, by weaving, knitting, braiding,stitching or similar etc . . . such that the same fabric width 6 whenrolled over itself from one of its edges 9 or 10 provides a ligament 1with free strands 4 corresponding to the central space of fabric width 6lying between the two picked side strips.

Evidently, several variants remain possible for making a ligament byconvoluted rolling of fabric width 6; for example a single-bundleligament 1 can be obtained by simply rolling either from edge 9 or fromedge 10 of fabric width 6 in concentric turns, the last turn beingstitched longitudinally to the preceding turn on these two end parts 2and 2′, as shown in FIG. 3 a and 3 b; ligament 1 can also be made byconvoluted rolling inwards from each opposite edge 9, 10, the twobundles when they meet being stitched longitudinally on the two endparts 2 and 2′ producing a ligament with two longitudinal axes that iscloser to the anatomy as shown in the diagram in FIG. 3 c.

Irrespective of how fabric width 6 is rolled, it is evident that theinner torsion imparted to each active strand 4 will, on median part 3 ofligament 1, give rise to a torque having the same value and samedirection on all the free strands, such that the ligament will naturallycoil around itself. Evidently it is possible, by reversing the directionof winding of spools 8 at the time of manufacture of fabric width 6, toproduce ligaments 1 whose natural torsion is oriented right (FIGS. 3 aand 3 c) or left (FIG. 3 b).

This essential characteristic of the ligaments of the invention will, ashas been seen, enable virtually ideal replacement of a torn biologicalligament by a prosthesis that is anatomically very similar. It hasalready been mentioned for example that the anterior cruciate ligamentundergoes approximately one eighth of a turn torsion when the knee movesfrom maximum flexion to maximum extension. However prosthetic anteriorcruciate ligaments are generally implanted with the knee in maximumflexion. It is therefore fully advantageous to implant a ligament thatis pre-coiled according to the method detailed above, which will providebiomechanical stability and isometric properties that are almost ideal.Also, this type of ligament whose active median part 3 is made up oflongitudinal strands 4 unconnected to one another in crosswise directionand pre-oriented as per right or left intrinsic coiling, is the only onewhich can not only prevent the torque stresses usually suffered byligaments which are the main cause of their rupture, but also reproducesthe phenomenon of gradual demand on fibres when the ligament is suddenlyplaced under stress. Mechanical laboratory tests, conducted on machinesreproducing knee kinetics, have shown that the ligaments such asdescribed in the present invention have a much higher resistance tofatigue than the ligaments of the prior art, including those describedby the applicant in patents FR-2 697 151 and FR-2 688 690.

According to another variant of the method of the invention, to obtain aright or left torsion of the active strands, it may be advantageous tocable twist two strands 11, 12 (which may each be made up of one orseveral filaments as mentioned above), that are torsion mounted, that isto say coiled around themselves in either direction in accordance withthe diagram given in FIG. 2 which shows a clockwise torsion S and ananti-clockwise torsion Z. It is easy to understand that each cable cordhas natural torsion in a given direction; in this case, all that isrequired is to replace active strands 4 of the previous method by acable cord 13 to obtain a fabric width 6 in the same manner which, whenrolled over from one of its edges or from both edges, will form aligament in accordance with either one of the diagrams in FIG. 3.

Here too the intra-osseous end parts 2 and 2′ of prosthetic ligament 1correspond, as seen above, to the knitted woven, braided, stitched orsimilar part of fabric width 6 using weft threads 5, 5′; also medianpart 3 of ligament 1, obtained with cable cords 13 that are free incrosswise direction and parallel to one another, will correspondprecisely to the intra-articular portion of the prosthesis whenimplanted in the joint.

In this respect, it is remarkable to observe that it is possible usingthe methods of the invention, to obtain ligaments that are fully adaptedto the ligament plasty under consideration; it can, for example, beplanned that the length of median portion 3 of ligament prosthesis 1,whether made with a bundle of strands 4 or cable cords 13, should be 30mm to act as a prosthetic anterior cruciate ligament, or 45 mm to act asa posterior cruciate ligament.

Needless to say, for each type of ligament prosthesis, the choice ofligament corresponding to a right or left joint must also be made,resulting, as mentioned above, from the clockwise or anti-clockwisetorsion of active strands 4 or cable cords 13.

Evidently any other method intended to impart torsion either to cablecords 13 or active strands 4 would come under the scope of the methodfor producing pre-oriented prosthetic ligaments in accordance with themain characteristic of the invention.

1. A method for producing a prosthetic ligament to replace a naturaljoint ligament, said method comprising: first applying a predeterminedtorsion individually to each of a plurality of strands, thereafterforming a bundle of said strands aligned in a parallel lengthwiserelationship, each strand having a median part and first and second endparts, securing said strands to each other at said respective first andsecond end parts to form first and second end sections of said bundle,said first and second end sections forming respective end members ofsaid prosthetic ligament, and having sufficient length to be embedded inbone insertion tunnels of a joint, and leaving said median parts of saidstrands unattached to collectively form a middle section of said bundle,said middle section of said bundle comprising a central member of saidprosthetic ligament approximating a length of an intra-articular portionof a natural ligament.
 2. The method for producing a prosthetic ligamentaccording to claim 1 in which said torsion is applied to said strands toa sufficient degree to create natural coiling of said prostheticligament by at least one eighth of a turn between said end members. 3.The method for producing a prosthetic ligament according to claim 1which said torsion is applied to said strands by unwinding each of saidstrands from a fixed spool, each said strand being pulled in an axialdirection relative to said spool, and perpendicular to a concentricwinding direction of said strand around said spool.
 4. The method forproducing a prosthetic ligament according to claim 1 in which saidapplied torsion has a same direction and value for each strand.
 5. Themethod for producing a prosthetic ligament according to claim 1 in whichsaid bundle in a pre-formed stage comprises a flat fabric of laterallyarrayed strands connected together side by side at their respective endparts, said fabric having exterior longitudinal edges formed fromrespective outermost strands, said fabric being adapted to be rolledlongitudinally in concentric turns beginning at a first exteriorlongitudinal edge and terminating at a second longitudinal edge to formsaid bundle, said second longitudinal edge being secured against saidfabric to prevent said bundle from unrolling.
 6. The method forproducing a prosthetic ligament according to claim 1 in which saidbundle in a pre-formed stage comprises a flat fabric of laterallyarrayed strands connected together side by side at their respective endparts, said fabric having exterior longitudinal edges formed fromrespective outermost strands, said fabric being adapted to be rolledlongitudinally inwards upon itself in concentric turns beginning at eachof said outermost strands such that two symmetrical bundles are formed.7. The method for producing a prosthetic ligament according to claim 1in which each of said strands comprises at least a pair of fiberswrapped around each other to form a twisted cable.
 8. The method forproducing a prosthetic ligament according to claim 7 in which adirection of twisting of said twisted cable is made in a clockwiseorientation to create a natural torsion of said prosthetic ligament foradaptation with a right joint.
 9. The method for producing a prostheticligament according to claim 7 in which a direction of twisting of saidtwisted cable is made in a counter-clockwise orientation to create anatural torsion of said prosthetic ligament for adaptation with a leftjoint.